This work consists of two projects which explore the nature of longitudinal mixing in the conducting airways and its relationship to alveolar ventilation. Experiments will be performed both in normal human subjects (project I) and in physical models (project II). The extent of mixing will be ascertained from the dispersion of the concentration break-through curve which results from the introduction of an inert gas bolus. In the physical models, we will also characterize the gas dynamics by using hot film anemometry. In project I, our previous data analysis techniques will be extended so that airway mixing and alveolar ventilation can both be inferred from the inert gas bolus-response. Then the relationship between mixing and ventilation will be determined in human subjects as a function of respiration frequency and bolus penetration volume. Helium, argon, and sulfur hexafluoride will all be used as inert gases in order to see if the mechanisms which dominate mixing and ventilation are sensitive to molecular diffusion coefficient. In project II, a 4-generation symmetric tube model and a 4-generation human lung cast will be constructed; a cast of the human larynx will be mounted at the proximal end of the models in order to simulate the upper airways. Both models will be placed on a test stand which generates inspiratory-like flows in the range from resting (0.4 l/s) to exercise (3.0 l/s) conditions. In addition to the inert gas bolus-response, we will measure the maximum axial velocity and turbulence intensity in each airway branch in order to determine the influence of the gas dynamics on airway mixing. We are particularly interested in the effect of turbulence generated by the larynx upon mixing processes which occur in downstream branches.